FR2491628A1 - RECEIVING CHANNEL AND COHERENT RADAR HAVING SUCH A RECEIVING CHAIN - Google Patents

Abstract

TO ENSURE THE PROTECTION OF THE RECEIVER AGAINST JAMMING, THIS RECEPTION CHAIN INCLUDES MEANS FOR PERFORMING A DOUBLE TRANSPOSITION 23, 26, BANDPASS FILTERING MEANS 22, 25, 27, 29 AND BLOCKING MEANS 30, 21 THE INVENTION APPLIES TO RADAR RECEPTION.

Description

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  The invention relates to a radar receiving chain and more

  particularly of coherent radar.

  In the radar receivers currently in operation, a reception antenna which receives the received signal of frequency FR, a local oscillator of frequency FL, and a mixer in which the received signal and the signal are transposed is essentially used.

  from the local oscillateuz. The local oscillator that generates the frequency

  transposition is usually "the slave" of an oscilla-

  power, for example a klystron or a magnetron, the de-

  pendence being performed using an automatic frequency control (C.A.F.). The output of the mixer provides a signal at the IF intermediate frequency which after filtering is processed by the

  detection and visualization means.

  Such receivers are highly exposed in a hostile electronic environment because of the interference resulting from both enemy jammers and other radars used in the same area of action. Indeed, for a given frequency of the oscillator

  local, the reception can be scrambled at the working frequency -

  EN; but also at the frequency of the image lateral band, as well as at all the harmonic beats of the intermediate frequency, that is to say the frequencies FR + (kJ) x (FI), where k is a relative integer, IkI ( absolute value of k) indicating the rank of

  the harmonic. It should be noted, however, that the effectiveness of the scrambling

  lage decreases as the rank of the harmonic increases.

  To eliminate the risks of interference on the image sideband (k - 1) and on its harmonics, it is known to use a double frequency transposition with a first transposition.

  high enough for the image to be filtered.

  The filtering is carried out, between the receiving antenna and the first mixer, by a band-pass filter or simply by a waveguide, the band of this filter being the band (BE)

  frequencies exploited on transmission.

  The band (BB) of frequencies likely to be scrambled is therefore equal to that of the transmission frequencies and the ratio

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R = BE is equal to 1.

  The invention proposes a reception chain for which the protection against interference is greatly improved by means that are easily achievable and do not present any particular technological difficulties.

  On the other hand, the invention makes it possible to increase the minimum of sen-

  detestable, to eliminate internal parasites, and to

  to catch the degradation of the reception in the moments which follow the emission. According to one characteristic of the invention, the coherent radar

  includes an emission chain that includes at least two

  coherent latters, the first providing a frequency signal

  and the second a variable frequency signal f2, at least one

  of which one of the entries is connected to the exit of the first

  the first input is connected to the output of the second oscillator via a second band-pass filter whose band is that in which f2 varies, and the output of which provides the signal of emission whose frequency f3 is equal to fi + f2, and a reception string which

comprises successively-: - -

  means for band-pass filtering of the received signal of frequency fr, these means being connected to the reception antenna and the filtering band corresponding to the frequency band in which the transmission frequency varies; first transposition means in which the signal coming from the first filtering means and the signal coming from the second oscillator of the transmission system are mixed, and which supply a signal of frequency fr - f2; amplification means constituted by a preamplifier; second band-pass filtering means; second transposition means in which are mixed the signal from the second filtering means and the signal coming from the first oscillator of the transmission chain and filtered by third bandpass filtering means centered on f1 and which provide a signal. frequency signal fr fl - f2; fourth means for band-pass filtering;

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  means for processing and displaying information contained in the signal derived from the third filtering means

  in order to improve the protection against jamming.

  According to a characteristic of the invention, the reception chain comprises between the reception antenna and the first signaling means.

  trage, a low-noise preamplifier that makes

  the insertion losses due to the first filtering means.

  According to another characteristic of the invention, the chain of re-

  ception comprises first blocking means connected between the first frequency multiplier and the fourth mixer, and second blocking means associated with the second preamplifier, the control inputs of the blocking means being connected to an output of the oscillator which provides a synchronization signal

  which triggers the blocking if, and only if there is a broadcast ..

  The objects and features of the present invention appear

  more clearly on reading the following description

  of an exemplary embodiment, this description being made in re-

  with a figure that represents an example of Doppler radar

consistent with the invention.

  This radar comprises an emission channel I and a chain of

reception 2.

  The transmission chain I comprises a very stable oscillator 10 which supplies a signal of frequency F0. The output of this oscillator is connected to the input of a formatting element 9 which comprises

  modulation means, coding means and / or means for

  spectrum depending on the type of program chosen. The output of the oscillator 10 is also connected to the input of a frequency multiplier circuit NI whose output provides an IF frequency signal. The output of the element 9 is connected to one of the inputs of a mixer 12; the other 12 entry is connected to

the output of the multiplier Il.

  The output of the multiplier circuit 11 is also connected to the input of a frequency multiplier circuit N2

  variable, the output of which provides a signal of frequency F2.

  The output of the multiplier circuit 13 is connected to one of the inputs of a mixer 14 whose other input is connected to the output

  mixer 12 which provides a frequency signal F0 + Fi.

  The output of the mixer 14 is connected to a transmitting antenna 15

  to which it provides a signal of frequency F3 equal to F0 + FI + F2.

  The mixer 12 due to its constitution, passes parasitic lines F0 + kFI frequency through the input which is connected to the output of the multiplier 11. To protect the radar against this internal parasite, is placed a bandpass filter 16 , centered on

  frequency and as close as possible between the output of the multi-

  For the same reasons, a band-pass filter 17, centered on the frequency F 2, is placed between the output of the multiplier 13 and the multiplier 13.

the mixer inlet 14.

  The emission frequency F3 is generated by mixing signals which are all generated from the frequency reference signal.

  F0 from the oscillator 10 and therefore the frequency signals

  Fl, F2 and F3 are consistent. The frequency transmission signal

  quence F3 varies in a frequency band of width a which depends

  of the variation of the ratio N2 of the circuit 13.

  In a particular embodiment, F0 = 60 MHz, FI = NI × F0 = 1800 MHz, F0 + F1 = 1860 MHz, F2 = N2 × F1, F2 varies from 7440 MHz to 8040 MHz, F3 = F0 + FI + F2, F3 varies from 9300 MHz to 9900 MHz and

A is equal to 600 MHz.

  The reception chain 2 comprises a reception antenna 20 which receives a signal of frequency FR between 9300 MHz and

9900 MHz.

  The antenna 20 is connected to the input of a preamplifier 21 low noise and gain AI whose output is connected to first bandpass filter means constituted by a bandpass filter 22 whose bandwidth A corresponds to the frequency band

  transmitted, that is to say 9300 MHz to 9900 MHz in the example shown below.

  above. The role of the amplifier 21 is to make negligible the insertion losses of the filter 22 in its contribution to the

  global noise. In the particular example, the AI gain of the amplifier

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  The controller 21 must be sufficiently high, of the order of 20 dB.

  The reception chain 2 comprises first transmission means

  position constituted by a mixer 23, one of whose inputs is connected to the output of the multiplier circuit 13 which provides the frequency signal F2. We thus realize a first transposition,

  the mixer 23 outputting a frequency signal FR - F2.

  The reception channel 2 comprises a second preamplifier 24 gain A2 whose input is connected to the output of the mixer 23 and

  whose output is connected to second pass filtering means.

  band consisting of a band-pass filter 25 of bandwidth 6 and centered on the frequency FO + FI. In the example, FO + FI is equal to 1860 MHz and e is chosen equal to 40 MHz, this value being the maximum tolerable value for rejecting the image of a beat at

more or less 60 MHz.

  The role of the amplifier 24 vis-à-vis the filter 25 is identical.

  to that of the amplifier 21 with respect to the filter 22.

  The reception channel 2 comprises second transmission means

  position constituted by a mixer 26, one of whose inputs is connected to the output of the filter 25 and whose other input is connected

  20 via a band-pass filter 29 at the output of the circuit.

  which provides the FI frequency signal. A second transposition is thus carried out, the mixer 26 outputting a frequency signal FR - F2 - Fi. The band-pass filter 29 is centered on the frequency F1 and makes it possible to eliminate any remaining parasite residue and more particularly that of frequency.

FO + Fi.

  The reception chain comprises third wire means

  band-pass filter constituted by a band-pass filter 27 of width d and centered on the frequency FO, the input of which is connected to the output of the mixer 26, and the output of which is connected to means 28 for processing and viewing information in the signal from the mixer 26. In the particular example, FO is equal to

MHz and d is equal to 4 MHz.

  Finally, the reception chain comprises first blocking means 30 constituted, for example, by a PIN diode attenuator and placed between the bandpass filter 29 and the mixer 26, and

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  second blocking means associated with the preamplifier 21.

  locking means are controlled by the same synchronization signal.

  sation from the element 9, so that there is blocking of the reception chain, at the input 21 and 30, whenever the transmission chain emits a pulse or in sequence, and, so that the blocking stops and the receiver is effective with

  its full sensitivity as soon as the show has stopped.

  Thanks to this reception chain with double transposition and triple successive filtering, the useful frequency F0 is free of parasitic frequencies. Indeed, the frequency of an eventual jammer can be effective only at the useful reception frequency and in the narrowest filter band, that is to say in our

  special case, the filter at 60 f.Hz band 4 MHz.

  The ratio R (R = BE frequency band exploited BB * frequency band likely to be scrambled is then equal to D. In our example, R = - = 150, which is 4

  corresponds to about 22 dB of protection.

  Apart from protection against jammers, the double

  transposition used in this reception chain allows to increase

  the sensitivity of the receiver, that is, to increase the

  detectable minimum value of approximately 3 dB.

  Furthermore, no internal parasite is to be feared thanks to

  efficient filtering performed by filters 16, 17 and 29.

  Finally, the sensitivity of the receiver is improved with regard to

  near areas by blocking the chain of

  during transmission times, which makes it possible to prevent the degradation of the sensitivity of the receiver during the instants which

follow the show.

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Claims (5)

  1. Coherent radar reception chain whose chain of   mission comprises at least two coherent oscillators, the first providing a signal of frequency f1 and the second a variable frequency signal f2, at least one mixer, one of whose inputs is connected to the output of the first oscillator via a first pass filter. a band centered on fi, the other input of which is connected to the output of the second oscillator via a second band-pass filter whose band is that in which f2 varies, and the output of which provides the transmission signal whose frequency f3 is equal to fl + ú 2, characterized in that it successively comprises first bandpass filtering means (22) of the received signal of frequency fr, these means being connected to the receiving antenna (20) and the filtering band corresponding to the frequency band in which the transmission frequency varies; first transposition means (23) in which the signal originating from the first filtering means (22) and the signal coming from the second oscillator of the transmission system are mixed, and which supply a signal of frequency fr - f2;   amplification means (24) consisting of a preamplifier   er; second means (25) for band-pass filtering;   second transposition means (26) in which are   the signal from the second filtering means and the signal from the first oscillator of the transmission chain, and filtered by   bandpass filtering means (29) centered on fl, and which   generate a frequency signal fr - fl - f2; fourth means (27) for bandpass filtering; means for processing and displaying information contained in the signal from the third filtering means;   to improve the protection against jamming.   2. Coherent radar receiving chain, according to the   cation 1, whose transmission chain (1) comprises a reference oscillator (10) of frequency FO whose output is connected firstly to the input of a shaping element (9) whose output is 249 1,628   connected to one of the inputs of a first mixer (12), and, on the other hand, to the input of a first frequency multiplier (11) of report NI whose output provides a signal of frequency FI equal to NI x F0 and is connected via a first bandpass filter (16) centered on IF to the second input of the first mixer (12) and to the input of a second frequency multiplier (13) of variable ratio N2 whose output provides a signal of frequency F2 equal to N2 x FI and is connected via a second passband filter (17) to one of the inputs of a second mixer (14) whose other input is connected to the output of the first mixer and whose output provides a signal of frequency F3 equal to F0 + FI + F2 and is connected to a transmitting antenna (15), characterized in that it comprises - a receiving antenna (20); a first bandpass filter (22) whose input is connected to the reception antenna and whose band of width A corresponds to the transmitted frequencies F3; a first mixer (23), one of whose inputs is connected to the output of the first filter (22) and whose other input is connected to the output of the second multiplier (13) of the transmission system - a first preamplifier (24) whose input is connected to the output of the first mixer; a second band-pass filter (25) whose input is connected to the output of the first preamplifier, centered on the frequency F0 + FI and of bandwidth 6;   a second mixer (26) one of whose inputs is connected to.   the output of the second filter (25) and whose other input is connected via a third band-pass filter (29) centered on F1 at the output of the first multiplier (11) of the transmission channel - a fourth bandpass filter ( 27) whose input is connected to the output of the second mixer, which is centered on the frequency F0 and whose bandwidth is equal to d; - and means of exploitation of information and visualization   (28) whose input is connected to the output of the fourth filter (27).   3. Coherent radar receiver chain according to the   cation 1 or claim 2, characterized in that it comprises, 2 4 9 16 28   between the receiving antenna and the first filtering means, a second preamplifier (21) low noise, so as to make negligible insertion losses due to the first filtering means. 1 4. Coherent radar receiving chain according to claim 2 and claim 3, characterized in that it comprises first blocking means (30) connected between the third filter (29) and the second mixer (26). , and second blocking means associated with the second preamplifier (21), the control inputs of the blocking means being connected to an output of   the shaping element (9) which provides a sync signal   which triggers the blocking if, and only if, there is an emission. 5. Coherent radar, characterized in that it comprises a chain   receiving device according to any one of the preceding claims.